Satellite cells of skeletal muscle are quiescent myogenic stem cells situated between the basement lamina and plasmalemma of the mature myofiber. Injury to the fiber induces the activation and proliferation of satellite cells whose progeny fuse to form new myotubes during regeneration. The overall goal of this research is to understand factors responsible for satellite cell proliferation. What stimulates satellite cell division during muscle growth and regeneration and what turns them off as the muscle reaches maturity? We have developed methods for preparing isolated single fibers with attached satellite cells from adult rats. Both fibers and satellite cells are viable for extended periods in culture. Preliminary experiments have shown that injured muscle releases a satellite cell mitogen that exhibits both source and target cell specificity. The mitogen stimulates proliferation of satellite cells and embryonic myoblasts in culture and when injected into neonatal rats. Satellite cells also proliferate in response to fibroblast growth factor, but not to other defined polypeptide mitogens. We will use viable single fiber-satellite cell units in culture to study the biological properties of the mitogen from muscle and to aid in its purification. We wish to determine whether the mitogen acts as a competence or progression factor, whether satellite cells exhibit chemotactic behavior to the mitogen and whether serum factors can modulate response of satellite cells to the mitogen. In addition, we will determine whether age-related impairment of regeneration results from weakened response of the satellite cells or lower production of mitogen by the muscle. Satellite cells gradually cease proliferating as the muscle matures. We will test the effect of mature fibers on proliferation of added myogenic cells obtained from embryos. Fiber innervation will also be examined as a possible modulator of satellite cell proliferation. Clinical management of traumatized muscle depends upon knowledge of how myogenic cells arise and what factors regulate their proliferation. Once we understand these basic principles of the formation of new muscle from damaged tissue, it may be possible to devise treatment to promote regeneration following injury or in neuromuscular disease.